1. Field of the Invention
The present invention relates in general to a vibration damping system for damping vibration of a power unit transmitted from a power unit to a vehicle body, and more particularly to a damper which is used in the vibration damping system and provided with two fluid chambers communicable through a communication passage with each other.
2. Description of the Prior Art
As a kind of vibration damping system for a power unit, an engine mounting device has been proposed in which the device includes a main body on which upper and lower mount rubbers are fixedly disposed, maintaining fluid-tight seal. The upper mount rubber is fixedly connected to an engine and defines therein a upper fluid chamber. The lower mount rubber is fixedly connected to a vehicle body and defines therein a lower chamber. The upper and lower chambers are communicated with each other through a communication passage formed through the main body. Additionally, the main body is provided with a control valve for controlling the area of the communication passage in response to engine operating conditions, so that the control valve is fully opened during idling. With this engine mounting device, the weight of the engine is supported by the upper mount rubber, and the upper and lower fluid chambers are communicated through the communication passage of a broadened area to lower the spring constant of the engine mount device thereby achieving vibration damping during idling or the like in which considerable vibration is transmitted from the engine or from the vehicle body.
Thus, the conventional engine mount device is intended to accomplish both engine weight support and vibration damping and arranged to control the area of the communication passage from a view point of the spring characteristics of the engine mounting device. As a result, even if the engine mount device is disposed in such a manner that fluid filled in the upper and lower chambes flowingly move upon movement of the engine in the direction of roll, the spring constant of the engine mount device in the roll direction unavoidably become larger because of addition of the spring characteristics for engine weight supporting, thereby making impossible to damp vehicle body vibration to a sufficient low level. In this regard, if the engine mount device is disposed in such a manner that no flowing movement of the fluid occurs against engine movement in the roll direction, an effect obtained by controlling the area of the communication passage is minimzed, so that the construction thereof hardly contribute to vibration damping.